1. Field of the Invention
The invention concerns a magnetic resonance apparatus.
2. Description of the Prior Art and Related Subject Matter
Magnetic resonance technology is a known technology to, among other things, acquire images of the inside of a body of an examination subject. In a magnetic resonance device, rapidly switched gradient fields that are generated by a gradient coil system are superimposed on a static basic magnetic field that is generated by a basic field magnet. Furthermore, the magnetic resonance device has a radio-frequency system that radiates radio-frequency energy into the examination subject to excite magnetic resonance signals, and acquires the magnetic resonance signals, on the basis of which magnetic resonance images are generated.
To generate gradient fields, appropriate currents are set in the gradient coils of the gradient coil system. The amplitudes of the required currents are up to more than 100 A. The current rise and fall rates are up to more than 100 kA/s. The basic magnetic field interacts with these temporally changing currents in the gradient coil to produce Lorentz forces on the order of 1 T, which lead to oscillations of the gradient coil system. These oscillations are propagated via various propagation paths at the surface of the magnetic resonance device. The mechanical oscillations are there transduced into sound vibrations that subsequently lead to an undesired noise.
From German OS 199 27 494, a tube-shaped gradient coil system for a magnetic resonance apparatus having rings at its ends similar to the tube cross-section and made from high-density materials. The rings are firmly connected with the gradient coil system for noise reduction.
From German OS 101 56 770, discloses a magnetic resonance apparatus with a gradient coil system in which an electrically-conductive structure is arranged and fashioned such that, at least within the imaging volume of the magnetic resonance apparatus, the magnetic field caused by a gradient field due to induction effects is configured geometrically similar to the gradient field. In an embodiment, at least one part of the structure is barrel-shaped as a component of a basic field magnet. Among other things, the gradient coil system can be fashioned without secondary coils since, due to the similarity of the magnetic field caused by the structure, the undesirable consequences of the gradient fields can be nearly completely controlled by a pre-distortion.
German PS 199 40 551 discloses a magnetic resonance apparatus with a basic field magnet with a two-part enclosure forming a bore. In the region of the enclosure in which the gradient coil system is attached, the enclosure is connected with a decoupling device that prevents the transfer of noise-generating oscillations from the gradient coil system to the enclosure of the basic field magnet.
From the subsequently published German OS 102 28 829, a magnetic resonance apparatus is known that has a basic field magnet with a bore that exhibits a shoulder (step). The gradient subassembly is clamped to this shoulder by means of a clamping element, such that a decrease in oscillation and a noise reduction are achieved.